Optical spectroscopy can be used to determine the concentration of chemical species in gaseous, liquid, and solid solutions. The amount of light absorbed by a particular chemical species is often linearly related to its concentration through Beer's Law, A=∈lc, where A is termed absorbance, ∈ is a constant specific to the chemical, 1 is the path length of light, and c is the concentration. A=log(I0/I), where I0 is the intensity of incident light, and I is the intensity of light after it has passed through a solution containing the chemical to be measured.
For nontransparent materials, including complex materials such as powders, tablets, natural materials (soil, agricultural products), blood, skin, and muscle, optical information can be collected via diffuse reflectance spectroscopy. In this setting, A=(I100/IR), where I100 is the light reflected from a 100% diffuse reflectance standard, the equivalent of the incident light, and IR is light reflected from the material under study. The concentration of a chemical component in one of these complex materials is related to A, though often not linearly. More sophisticated mathematical techniques, such as, for example, partial least squares regression and other multivariate calibration methods are used to determine a relationship between concentration and absorbance. Once these calibration models are derived, they can be used to determine chemical composition by measuring absorbance in the transmittance or reflectance mode.
In the laboratory, it is relatively straightforward to measure the absorbance A. One method uses a temperature controlled dual beam spectrograph. The sample solution is placed in one chamber, and the amount of light transmitted through the sample is measured. The solvent alone is placed in the second chamber where the incident light is measured. The ratio is determined electronically and the absorbance reported.
Laboratory spectrographs are generally large, expensive, and not portable. Recently, smaller spectrographs have been introduced that allow absorbance measurements to be performed in the “field,” meaning that the spectrometer equipment can be brought to the sample, rather than requiring that the sample be brought to the lab.
Spectroscopic measurements are now commonly made of agricultural products, in the ocean and in forests, on manufacturing production lines, and on the human body. Most of these measurements are made in the reflectance mode in which a fiber optic based sensor directs light to the sample and measures light reflected back from the sample. Field measurements are often made in an ongoing, continuous manner, to observe temporal changes of a quantity measured spectrographically. Sometimes these measurements are made in a “hostile” environment, where it is difficult to make electrical measurements because these sensors experience interference (e.g., in an MRI machine) or degradation (e.g., due to a smoke stack or waste water).
Usually these measurements are made by first collecting light from the 100% reflectance standard, storing that number, then attaching the sensor to the sample and collecting a series of spectra. The initial reference measurement of the 100% reflectance standard is used to calibrate absorbance from all subsequent sample spectra. This process, unfortunately, can introduce significant error, especially when the target absorbance changes are small and present in a complex, interfering chemical mixture, such as those studied in the field. Over time there can be changes in the lamp output and the detector sensitivity, which alter the intensity and spectral temperature of light impinging on the sample. If these changes are not detected and corrected in real time in the absorbance calculation, the measured value of A will be erroneous, and an accurate concentration of the measured quantity cannot be made.